While I am not a fan of McDonald's (except for it's burnt, watery coffee) the reason for the higher prices is the following:
All persons employed "en blanco" in Argentina in retail stores belong to the Retail Employees Union, known as SEC (Sindicato de Empleados de Comercio). They get a monthly salary, plus full health benefits, unlimited sick days, a yearly bonus equivalent to a month's salary, paid study days (if they are in school), overtime pay, and if they are fired paid an indemnization of 1 months salary for each year they have worked for the company. These higher costs are the reason why generally all retail prices are higher in Argentina. McDonalds employees in the USA get a total of approximately $8 U$S per hour, no health benefits, no vacation pay, no bonus, just the $8 per hour and that is hard if not impossible to live on if you don't live at home with your parents and have 0 expenses.